The present development is a process for extrusion coating a film substrate with a microlayered extrusion coating or for laminating two layers of film substrates with a microlayered extrudate. The present development also includes a microlayered extrudate for use in extrusion coating or extrusion lamination, and the resulting film products formed using the microlayered extrudate. The microlayered extrudate of the present invention comprises a plurality of polymeric microlayers, wherein each layer has a thickness of less than five microns, and preferably less than one micron, and alternating tie layers sandwiched between polymeric boundary layers which are further sandwiched between polymeric skin layers. The resulting product defines a laminate comprising a first substrate/the microlayered extrudate/a second substrate.
Multilayer barrier films are widely used in the packaging industry. Some representative applications include wraps for meat and cheese, and packaging for snack foods, cereals, crackers, cookies, and baking mixes. These multilayer barrier films may comprise a single substrate with a barrier extrusion coating, or two or more substrates may be held together by bonding layers produced through adhesive lamination in polymer layers, such as the bonding produced through extrusion lamination. Failure to achieve good adhesion with the extrusion coating or extrusion lamination can result in delamination which can cause loss of product integrity and, therefore, loss of protection for the food through loss of a moisture and/or oxygen barrier, or may allow the package to tear and expose the food.
As is known in the art, the selection of the layers for any particular multilayer barrier film is dependent on the intended end-use application. Polyethylene (PE) resins, such as low density polyethylene (LDPE) and high density polyethylene (HDPE), are widely used for their ability to provide essential properties such as structural integrity, puncture resistance, and abrasion resistance. Further, PE is a low cost bonding material that can be used with most polymer films. Ethylene-vinyl alcohol copolymers (EVOH) are widely used as barrier layers to prevent the passage of oxygen. Polyamide (PA) films provide moderate oxygen barrier protection. High density polyethylene (HDPE) is commonly used to provide moisture barrier protection. The adhesion layer materials are selected for their ability to adhere to the neighboring layers. For example, mixtures of functionalized polyolefins are often used to facilitate adhesion of the non-polar polyethylene layers to the polar barrier layers.
Extrusion coating is a versatile, low cost technique that has long been applied to multilayer films. The extrusion coating process involves extruding an accurately metered quantity of molten resin from a slot die directly onto a substrate, which is presented as a moving web. The coating and web are then pulled into a nip between a cooling roll and a pressure roll. The pressure between these two rolls forces the extruded molten resin onto the substrate web surface, which is moving at a speed faster than the extruded resin, thereby drawing the molten resin to the required thickness. The cooling roll chills the molten resin into a solid state which then releases from the chrome plated cooling roll.
Extrusion laminating is a process similar to extrusion coating except the extruded hot molten resin, or multiple resins, are fed into the nip between two separate substrates. When the resin cools, it bonds the substrates together. Skin layers of copolymers and terpolymers may be coextruded with the resin to provide improved bonding.
For the extrusion coating and extrusion lamination processes, common substrates are paperboard, corrugated fiberboard, paper, aluminum foils, cellulose or plastic films. The substrates may be multilayered, or metallized, or may include printing, or any combination thereof. For example, the extrusion lamination process may be used to bond an outer printed film substrate and an inner sealing film substrate to form a two-film flexible laminate for packaging. Resins are selected based on the probability of achieving good adhesion. Exemplary resins include polyethylene, LDPE, linear low density polyethylene (LLDPE), binary blends of PE, ethylene vinyl acetate copolymers and polypropylene. Optionally, polymer resin additives, such as maleic anhydrides, acrylate-based resins, or other bond-enhancing resins, can be added to the extrusion resin to improve bonding to specific films.
Although the resins used for producing films and the resins used for extrusion coating and extrusion lamination fall within the same broad chemical classes, there are distinct differences. For the extrusion coating and extrusion lamination processes, the optimal resin must provide sufficient oxidation for bonding to the substrate surfaces. Otherwise, delamination of the substrate from the bonding resin may occur. Conversely, resins selected for film production normally include antioxidants to prevent oxidation. For the extrusion coating and extrusion lamination processes, the resin must have adequate melt flow. For example, a melt flow index equal to or greater than 7 is typical for extrusion lamination for flexible packaging, and a melt flow index equal to or greater than 22 is also optimal for caulkability. By contrast, for film production, resins preferably have melt flow indices less than 5. For the extrusion coating and extrusion lamination processes, the melt temperature of the extruding resin that contacts the substrate surface also affects oxidation. For optimal oxidation the resin preferably has a melt temperature of at least about 585° F., but preferably between about 600° F. and about 620° F., although at these temperatures the melt curtain stability and profile across the web in the transverse direction has some gauge, or thickness, variation. For film production, such as polypropylenes and polyethylenes, the resins selected normally have melt temperatures below about 475° F.
Over the past decade, microlayered films have gained a commercial following. Microlayered films are multilayered films formed by cast coextrusions wherein the film structure comprises one or more sections comprising from less than ten layers to over a thousand layers, each layer being from about 0.01 to about 5 microns thick, and wherein the microlayer section is normally sandwiched between two bulk film layers or is adhered to one surface of a traditional bulk film layer. The layers may be formed in a coextrusion feedblock by splitting and stacking a small number of input melt streams, or by sequential layering to create alternating microlayers which then come together and enter an extrusion die as an extrudate comprising the alternating microlayers. Optical films typically use fewer polymers, and more and thinner layers; barrier films use fewer, thicker microlayers and more polymers.
Some exemplary microlayered films are taught in U.S. Published Application 2011/0229722 A1, U.S. Published Application 2011/0039098 A1, U.S. Pat. No. 8,080,310, and U.S. Pat. No. 8,012,572. U.S. Published Application 2011/0229722 A1 (ascribed to Rivett et al.) describes a multilayer oxygen barrier film that includes at least one bulk layer and a microlayer comprising ethylene vinyl alcohol. Thicknesses of each layer of the microlayer may range from about 0.0001 mils to about 0.1 mils. The film of the '722 application is produced using a system for coextruding a plurality of fluid layers, such as taught in U.S. Published Application 2010/0072655 A1. To produce the film of the '722 application, a microlayered fluid mass is merged with a first bulk layer within a die thereby forming a multilayer film as a relatively thick extrudate, which comprises the bulk layer and the microlayered section as solidified film layers resulting from the fluid (molten) polymer layer and the microlayered fluid mass with the die.
U.S. Published Application 2011/0039098 A1 (ascribed to Forloni et al.) describes a gas barrier film for use in packaging. The film of the '098 application comprises thermoplastic (co)polyamides and ethylene vinyl alcohol copolymers which are split and stacked to produce a microlayered structure. The microlayered structure may be incorporated as a core layer of a film by using, for example, a five-layer feedblock that allows the microlayered melt stream to pass into the feedblock together with a first substrate layer and a second substrate layer and two intermediate adhesive layers, wherein one adhesive layer is positioned between the microlayers and the first substrate layer and the second adhesive layer is positioned between the first and second substrate layers.
U.S. Pat. No. 8,080,310 and U.S. Pat. No. 8,012,572 (both issued to Ramli et al.) describe heat-shrinkable films. The film of the '310 patent comprises a microlayer structure comprising ethylene vinyl alcohol and polyamides; the '572 patent teaches a multilayer film similar to that taught in the '310 patent, but does not expressly include EVOH in the microlayer structure. The '310 patent teaches a film in which the microlayer section is positioned at an exterior surface of the film, such that one of the microlayers forms an outer layer for the resultant heat-shrinkable, multilayer film. As with the film of USPA '722, the microlayered fluid mass is merged with a first bulk layer within the die thereby forming a multilayer film as a relatively thick tape extrudate.
Although the microlayered films taught in the prior art include films with microlayer sections positioned in the core and on the exterior of a film, the microlayered films of the prior art require that the microlayers be coextruded with the bulk layer(s) of the film. For the packaging industry, this is extremely limiting with respect to the selection of substrates that may be employed if the benefits of microlayering are to be recognized.
In response to the above identified problems, it would be desirable to develop a method wherein a microlayered material section could be produced and bonded to bulk film layers using standard extrusion coating or extrusion lamination techniques. Further, it would be desirable for the microlayered material section to provide oxygen barrier benefits, moisture barrier benefits, improved opacity, or a combination thereof, to the resulting multilayered laminate.